The polymerisation of olefins in the gas phase in fluidised or similar reactors is well-known and commercially operated. In a typical fluidised bed process, for example, a bed of growing polymer particles is maintained in a fluidised state in a vertically orientated cylindrical reactor by means of an ascending gas stream (fluidising gas). Gas exiting the top of the fluidised bed reactor is cooled and recycled to the base via a recycle loop. Fresh monomer(s) are added to replace those consumed in the reaction whilst produced polymer particles are removed from the bed.
GB 1,413,613, for example, describes a process for the dry polymerisation of olefins in a fluidised bed reactor. Two possible configurations are described to address the potential problem of small solid particles, known as “fines”, being entrained from the reactor with the ascending gas stream. The first of these is the use of what is referred to in GB 1,413,613 as a “tranquilisation chamber” which is an area of larger cross-section compared to the reactor and provided above the reactor which acts to reduce the velocity of the fluidising gas, allowing solid particles which might otherwise exit the top of the fluidised bed to fall back to the bed. The second is the use of a cyclone to separate the entrained solids for subsequent return to the reactor.
Of the two options disclosed in GB 1,413,613, that referred to as the tranquilisation chamber, which may alternatively and, in fact, is more generally referred to as a disengagement zone or velocity reduction zone, has become the commonly used means for reducing entrainment from fluidised bed reactors. However, despite the use of a disengagement zone, it is common for significant quantities of solids to remain entrained in the fluidising gas exiting the disengagement zone. Such solids may foul components of the line through which the fluidising gas is recycled. To avoid this problem it is therefore known to also use a cyclone in addition to the disengagement zone to separate the entrained solids from the fluidising gas exiting the disengagement zone.
Such processes are described, for example, in U.S. Pat. No. 4,882,400 and U.S. Pat. No. 5,382,638.
U.S. Pat. No. 4,882,400, for example, discloses an apparatus for the gas phase polymerisation of olefins in a fluidised bed reactor comprising a disengagement chamber and a cyclone which acts to separate entrained solids from the fluidising gas exiting the reactor, for recycle to the fluidised bed. According to U.S. Pat. No. 4,882,400, the solids should be recycled to the reactor, below the disengagement chamber but above the top of the fluidised bed. One problem of this process is that it is necessary for the fluidised bed to operate below the height of the cylindrical reactor section.
U.S. Pat. No. 5,382,638 also discloses an apparatus for the gas phase polymerisation of olefins in a fluidised bed reactor comprising a disengagement chamber and a cyclone which acts to separate entrained solids from the fluidising gas exiting the reactor, for recycle to the fluidised bed. According to U.S. Pat. No. 5,382,638, the solids should again be recycled to the upper portion of the reactor body, but in this case may be introduced either above or into the top of the fluidised bed. According to U.S. Pat. No. 5,382,638 recycle of the solids to the lower portion of the bed is less favoured.